Annual Motion Annual Motion ecliptic z As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. the apparent path of the Sun through the sky z The Sun circles the celestial sphere once every equinox year. where the ecliptic intersects the celestial equator solstice where the ecliptic is farthest from the celestial equator zodiac the constellations© 2004 Pearson which Education Inc., lie publishing along as the ecliptic © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley Annual Motion The Cause of the Seasons z The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane. z Therefore, the celestial equator is tilted 23.5° to the ecliptic. z As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator. z Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Sun! Addison-Wesley Addison-Wesley Axis tilt causes uneven heating by Seasonal Change in Sun’s sunlight throughout the year. Altitude z The “Figure 8” shows Sun at same time each day over a year. © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley 1 Seasonal changes are more When is summer? extreme at high latitudes Path of the Sun on the summer solstice at the z Although the solstice Arctic Circle which occurs around June 21 is considered the first day of summer. z It takes time for the more direct sunlight to heat up the land and water. z Therefore, July & August are typically hotter than June. © 2004 Pearson Education Inc., publishing as © 2004 Pearson EducationImage: Inc., publishinghttp://www.beachchamber.com/newwebsite/photos/ as Addison-Wesley Addison-Wesley Precession of the Equinoxes Why doesn’t distance matter? • The Earth’s axis precesses (wobbles) like a z Small variation for Earth — about 3% (but distance top, once about every 26,000 years. does matter for some other planets, notably Mars and • Precession changes the positions in the sky of Pluto). z Surprisingly, seasons are more extreme in N. the celestial poles and the equinoxes. hemisphere, even thought Earth is closer to Sun in S. ⇒ Polaris won't always be the north star. hemisphere summer (and farther in S. hemisphere winter) — because of land/ocean distribution ⇒ The spring equinox, seen by ancient Greeks in Aries, moves westward and is now in Pisces! © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley Sun sign vs. Tropical Sign Horoscope is about one month off! z Tilt of Earth’s axis towards or away from z Consider extreme case: in 13,000 years, sun’s axis will Tilt of Earth’s axis towards or away from rotate from towards the sun to away from sun. sun determines Season z In July, Sun is in conjunction with Leo z Calendar is designed to keep same z In 13,000 years, the sun will be in the same place in its orbit, and in conjunction with Leo, but the calendar will season in the same month…it accounts for say that it is January. precession z The Zodiac/Horoscope dates were established by Ptolemy, about 100 AD. Since then the Axis has z Precession changes the place on the wobbled 2/27 of a complete circle—about a months Earth’s orbit where a season and month worth if you are counting. z Astrologers differentiate between the Tropical sign (by occurs date) and the Sun sign (by position)…good luck! z It is impossible to keep Season, month, and position in orbit the same…so: © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley 2 The Local Sky To pinpoint a spot in the local sky: Specify altitude –the angle above zenith the ground and the point directly above you -azimuth is the number of degrees east horizon of North along the horizon. all points 90° from the zenith Altitude the angle above the horizon, Azimuth—angle from North horizon meridian due north horizon© 2004 Pearson⇒zenith Education Inc., publishing ⇒due as south horizon © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley Coordinates on the Earth Measuring the Sky z Latitude: position north or south of equator equator We measure the sky in angles, not distances. z Longitude: position east or west of prime meridian (runs through Greenwich, England) z Full circle = 360º z 1º = 60 arcmin = 60’ z 1 arcmin = 60 arcsec = 60” © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley Measuring Angles in the Sky Venus, Jupiter, Crescent Moon What is the The angle between Venus and Jupiter? 5 degrees! http://www.shoestringastronomy.org.uk/photo/conj/moon_venus_jupiter.jpg © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley 3 Phases of the Moon Lunar Motion Phases of the Moon’s 29.5 day cycle • new • crescent • first quarter waxing • gibbous • full • gibbous • last quarter waning © 2004 Pearson Education Inc., publishing as • crescent © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley Why do we see the same face? Earth and Moon from space Rotation period = orbital period This won’t play on the web, but see PlanetTales! © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley Eclipses z Moon’s orbit tilted 5° to ecliptic plane z Crosses ecliptic plane only at the two nodes z Eclipse possible only when full/new occur near z The Earth & Moon cast nodes shadows. z When either passes through the other’s shadow, we have an eclipse. z Why don’t we have an eclipse every full & new Moon? © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley 4 Solar Eclipse Lunar Eclipse © 2004 Pearson Education Inc., publishing as © 2004 Pearson Education Inc., publishing as Addison-Wesley Addison-Wesley 5.